1. Field of the invention
The present invention relates to a releasing agent for powder molding and a process for producing a molded article using said releasing agent. More particularly, the present invention relates to a releasing agent for powder molding, which comprises a polymer having (a) a silyl group having methyl group or fluorine atom-containing substituent of 1-20 carbon atoms as a side chain or (b) (poly)dimethylsiloxanyl group (in this specification, "(poly)dimethylsiloxanyl" means "dimethylsiloxanyl" or "(poly)dimethylsiloxanyl") having a methyl group or a fluorine atom-containing substituent of 1-20 carbon atoms at the terminal, as well as to a process for producing a molded article using said releasing agent.
2. Prior Art
There were recently developed various molding processes and coating processes each using a powdery synthetic resin. Typical of these processes include a rotational molding process, a slush molding process, a fluidization dip coating process and an electrostatic coating process. All of these molding and coating processes have an advantage in that they can produce an intended pattern very well. Therefore, the molds used in these molding and coating processes have a very complex shape with fine projections and depressions. However, the molded articles obtained with such molds mesh tightly like a dropped anchor with the fine projections and depressions of the molds, making it difficult to peel them from the molds with complex shape. Hence, there has conventionally been adopted a method of imparting lubricity to the interface between mold and molded article, and there has been widely used, as a releasing agent, an internal lubricant which is added to a molding material, or an external lubricant which is coated on the surface of the mold.
These conventional releasing agents for powder molding are intended to allow the interface between mold and molded article to have lubricity. However, the internal lubricant which is added to a molding material, is superior in handling but very easily bleeds out onto the surface of molded article. Reducing the bleeding by improving the compatibility of the internal lubricant with a resin (e.g. vinyl chloride resin) which is a main component of molding material, invites retention of the internal lubricant within the molded article and resultantly no migration of the lubricant to the interface between mold and molded article; therefore, the lubricant does not achieve the intended effect. On the other hand, the external lubricant which is coated on a mold for reduction in peeling strength, has poor compatibility with both the mold and a molded article and accordingly induces repellence between the mold and the molded article; thus, it has an excellent releasing effect. However, when the molded article is peeled from the mold, the external lubricant is repelled by the surfaces of the mold and the molded article because of its poor compatibility with them, and remains on the surfaces in liquid or solid spots. This requires, after molding, cleaning of the molded article and the mold to remove the external lubricant remaining thereon. Cleaning of the mold in particular, after each molding operation, reduces work efficiency; in order to avoid low work efficiency, mold cleaning is actually effected once per several molding operations. Such continuous use of mold without cleaning, however, causes the oxidation and/or decomposition of external releasing agent and results in gradual cloudiness in spots of the mold's mirror surface. Further progress of this phenomenon causes coverage of the fine projections and depressions of the mold by oxidized and/or decomposed external lubricant. Since in powder molding, the pattern of a mold is reproduced precisely, the coverage of the projections and depressions or the formation of cloudy portions implies that these covered or cloudy portions are also reproduced as such in the molded article.
The objects of the present invention are to provide a releasing agent for powder molding, which is free from the above-mentioned drawbacks of the conventionally known releasing agent for powder molding, and which uses a volatile solvent type polymer capable of forming a film of larger contact angle than the conventional releasing agents and accordingly of good releasability, as well as to provide a process for producing a molded article using said releasing agent.